Differenze Ultrastrutturali Di Alcune Specie Di Trebouxia Poste in Condizioni Di Illuminazione Differenti

Abstract

Ultrastructural changes in some species of « Trebouxia » under different light conditions. — Some species of the phycobiont alga Trebouxia (Tr. decolorans and Tr. albulescens), both isolated and grown on synthetic medium and still in the lichen, were examined in order to study the effect of light on the plastid ultrastructures. The species isolated from Buellia punctata and Xanthoria parietina were very sensitive to light condition and lost their chlorophyll content quickly. Striking ultrastructural changes were found in the algae grown under small light intensities and those which become achloric owing to strong light. In the latter, modifications of the Iamellar System were observed. The disappearance of Chlorophyll pigments was followed by a reduced electron density of the whole Iamellar system, as if were lacking the Iipidic compounds which are usually present and absorb fixators and dyers, thus allowing a good view. On the contrary, normal light conditions did not affect cultures of Trebouxia humicola, a free living alga. In the chloroplasts of the phycobiont species, unlike in the free living alga, grana were very close and sometimes formed very thick masses towards the edge of the chloroplast. It could not be ascertained whether such changes corresponded to a different composition of the lipoproteic compounds of the lamellar system.

Xanthoria parietina could grow in very lighted environments with no damage of the algae present in its thallus. The lichen thalluses, under different light conditions, showed very different colourings: the overlighted ones were rusty-red and the shadowed ones deep green. The chlorophyll content of the lichen thalluses with various shades (table 1) were very similar. The ultrastructural changes induced by strong light intensities in the phycobiont algae, kept in the lichen, were very small in respect of those observed in the same algae isolated and grown on synthetic medium and concerned the Iamellar system and the pyrenoid, above all. The rusty-red lichen showed a great number of stromatic lamellae, often with a parallel trend, so as to simulate a Iamellar system not organized in grana and often presented groups of lamellae concentrically arranged. In the pyrenoid of the algae from rusty-red thalluses, compared with the green ones, a much greater number of electron dense masses was observed, which are very thick and occupy the whole stromatic portion of the pyrenoid. But the Chlorophyll content did not decrease. Unlike the results of PEVELING, we noted that the electron dense masses (cited by the Author as « osmiophilic plastoglobules) were visible even after fixation with permanganate; the different numbers of these globules might depend on environmental factors. The phycobiont alga, when in the lichen thallus, could perhaps support strong light intensities, because pigments or compounds formed with the mycobiont or by it alone prevented the photooxidation of chlorophyll. Hypothetically a relationship might exist between the sensitivity of the phycobiont algae to light intensities and the content in antraquinonic pigments in the lichen thallus. But also using filters with absorption maxima similar to those of these pigments, the « in vitro » cultures of the phycobiont algae became achloric in the same time as the control ones.

Some Authors had found in Trebouxia humicola a different relationship between Chlorophyll pigments and carotinoids from that observed in the phycobiont species and had ascribed to it the greater resistence to strong light of the free living alga. Pigments or other substances present in the mycobiont can have a protective action on the Chlorophyll content and on the ultrastructures. In the phycobiont algae the resistence to strong light might be explained by an exchange of compounds between mycobiont and phycobiont, ending with the structural changes of the pyrenoid.